1. Field of the Invention
The invention relates to a liquid crystal composition and a liquid crystal display device containing the composition.
2. Related Art
A liquid crystal display device (which is a generic term that includes a liquid crystal display device, a liquid crystal display panel and a liquid crystal display module) utilizes optical anisotropy, dielectric anisotropy and so forth of a liquid crystal composition, and as an operating mode of the liquid crystal display device, such various modes have been known as a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, and so forth. Among these display modes, it has been known that an ECB mode, an IPS mode, a VA mode and so forth are capable of being improved in viewing angle while the conventional modes, such as a TN mode, an STN mode and so forth, have a problem therein. A liquid crystal composition having a negative dielectric anisotropy can be used in a liquid crystal display device of these modes. A liquid crystal compound having 2,3-difluorophenylene contained in the liquid crystal composition having a negative dielectric anisotropy is being studied (as described, for example, in Japanese Patent No. 2811342 and No. 1761492). A liquid crystal composition having a negative dielectric anisotropy capable of being used in the liquid crystal display device has also been studied (as described, for example, in DE 19 607 043, JP 2004-532344 (International Publication No. 02/99010), and JP H10-176167 A/1998).
A fluorine-replaced liquid crystal compound and a liquid crystal composition containing the compound are disclosed in JP H07-053432 A/1995. However, the technique disclosed in JP H07-053432 A/1995 considers only a liquid crystal compound having a positive dielectric anisotropy but fails to study a liquid crystal compound having a negative dielectric anisotropy. A liquid crystal composition having a combination of a liquid crystal compound having 2,3-difluorophenylene and a non-fluorine-replaced liquid crystal compound is disclosed in DE 19 607 043, JP 2004-532344 (International Publication No. 02/99010), and JP H10-176167 A/1998. However, the compound contains a non-fluorine-replaced liquid crystal compound not having a negative dielectric anisotropy, and there are some cases where the compound does not have a negatively large dielectric anisotropy. A liquid crystal composition having a combination including a mono-fluorine-replaced, which analogous to the first component of the invention, is disclosed in DE 19 607 043 and JP H10-176167A/1998. However, the compounds disclosed in the examples thereof have a negatively small dielectric anisotropy, and the minimum temperature of a nematic phase has not been clarified.
The liquid crystal compound is a generic term for a compound having a liquid crystal phase such as a nematic phase, a smectic phase and so forth, and also for a compound having no liquid crystal phase but being useful as a component of a composition. The content ratio of the component is calculated based on the total weight of the liquid crystal composition. The liquid crystal compound herein is a compound represented by formula (A). The compound may be an optically active compound:

In formula (A), Rx and Ry are independently hydrogen, alkyl, alkoxy, alkoxyalkyl, alkoxyalkoxy, acyloxy, acyloxyalkyl, alkoxycarbonyl, alkoxycarbonylalkyl, alkenyl, alkenyloxy, alkenyloxyalkyl, alkoxyalkenyl, alkynyl, alkynyloxy, cyano, —NCS, fluorine or chlorine. These groups have 10 or less carbons. In the group having 1 to 5 carbons, arbitrary hydrogen may be replaced by fluorine or chlorine, and the total number of the replaced fluorine and chlorine is 1 to 11. In formula (A), ring B is 1,4-cyclohexylene, 1,4-phenylene, pyrane-2,5-diyl, 1,3-dioxane-2,5-diyl, pyridine-2,5-diyl, pyrimidine-2,5-diyl, decahydronaphthalene-2,6-diyl, 1,2,3,4-tetrahydronaphthalene-2,6-diyl or naphthalene-2,6-diyl. In ring B, arbitrary hydrogen may be replaced by fluorine or chlorine. In ring B, the total number of the replaced fluorine and chlorine is 1 to 4. In the 1,4-phenylene, arbitrary one or two hydrogens may be replaced by cyano, methyl, difluoromethyl or trifluoromethyl. In formula (A), Y represents a single bond, —(CH2)2—, —COO—, —OCO—, —CH2O—, —OCH2—, —CF2O—, —OCF2—, —CH═CH—, —CF═CF—, —(CH2)4—, —(CH2)3—O—, —O—(CH2)3—, —CH═CH—(CH2)2—, —(CH2)2—CH═CH—, —(CH2)2CF2O—, —OCF2(CH2)2—, —(CH2)2COO—, —(CH2)2OCO—, —COO(CH2)2—, —OCO(CH2)2—, —CH═CH—COO—, —CH═CH—OCO—, —COO—CH═CH— or —OCO—CH═CH—. In formula (A), n represents 1, 2, 3 or 4.
A liquid crystal display device having such a display mode as an IPS mode and a VA mode still has a problem as a display device as compared to CRT, and is demanded to be improved in characteristics thereof. The liquid crystal display device driven in an IPS mode or a VA mode is constituted mainly by a liquid crystal composition having a negative dielectric anisotropy, and in order to further improve the characteristics, the liquid crystal composition preferably has the following characteristics (1) to (5), i.e., (1) a wide temperature range of a nematic phase, (2) a low viscosity, (3) a suitable optical anisotropy, (4) a large absolute value of dielectric anisotropy, and (5) a large specific resistance.
The temperature range of a nematic phase relates to a temperature range where the liquid crystal display device is used, and a liquid crystal display device containing a liquid crystal composition having a wide temperature range of a nematic phase as in the item (1) has a wide temperature range where the liquid crystal display device can be used. A liquid crystal display device containing a liquid crystal composition having a small viscosity as in the item (2) has a short response time. A liquid crystal display device having a short response time can be favorably used for displaying a moving image. Furthermore, upon injecting the liquid crystal composition into a liquid crystal cell of the liquid crystal display device, the injection time can be reduced to improve the workability. A liquid crystal display device containing a liquid crystal composition having a suitable optical anisotropy as in the item (3) has a large contrast. A liquid crystal display device containing a liquid crystal composition having a large absolute value of dielectric anisotropy as in the item (4) reduces a threshold voltage, decreases a driving voltage, and reduces an electric power consumption. A liquid crystal display device containing a liquid crystal composition having a large specific resistance as in the item (5) increases a voltage holding ratio and increases a contrast ratio. Therefore, such a liquid crystal composition is preferred that has a large specific resistance in the initial stage and has a large specific resistance even after using for a long period of time.